A reciprocating compressor operates to compress a refrigerant fluid by use of a piston operating in a cylinder. The reciprocating motion of the piston in the cylinder compresses the refrigerant. The reciprocating motion of the piston is due to a crankshaft and a rod, which converts the rotary motion of an electric motor to reciprocating motion. The piston is coupled to the crankshaft by a connecting rod. The connecting rod is attached to the piston by a piston pin or wrist pin, which is inserted into the piston. Activation of the electric motor causes the crankshaft to rotate, which in turn moves the connecting rod and piston as the crankshaft rotates.
In hermitically scaled reciprocating compressors, pistons are light in weight, typically being made from an aluminum alloy. While a piston can be made by a variety of methods, such as forging, they are made significantly more economically by die casting. Die casting forces molten metal into a mold cavity under high pressure. The molten metal is poured into a shot chamber, and a plunger forces the molten metal into a die cavity. After the molten metal solidifies, the die is opened and the casting and excess metal are removed from the die. The advantages of die casting include dimensional tolerance accuracy, excellent surface finish and castings of high strength. The process also enables a high production rate. Thick sections should be avoided, as these sections require additional time to solidify, thereby adversely affecting the strength of the casting. Die casting also allows for accurate coring and casting of inserts. Of course, the casting design must be such that the mold cavity and cores allow the casting to be ejected. Currently, the piston castings include coring which defines the geometry of the castings, including the size and location of a wrist pin or piston pin.
The coring of the die casting to locate a wrist pin eliminates subsequent manufacturing operations. However, the aperture produced by the coring creates other problems that are desirable to overcome. First, the location of the coring produces a piston in which the wrist pin aperture is in a fixed location. Thus, in order to change the location of the wrist pin aperture to vary the stroke of the piston, it is necessary to produce different castings with a core located at a different position along the length of the piston. It would be desirable to produce a single casting in which the wrist pin can be located at a varying position along the length of the piston in order to produce a desired piston stroke rather than produce a series of such castings with varying wrist pin locations.
Additionally, in existing die castings, the core used in the casting is about 0.030 inches smaller than the finished diameter. This produces an aperture at diametrally opposed locations along the diameter of the casting through its thickness. Of course, this reduces the amount of molten material that must be provided to the casting. The cooling rate and the injection pressure can be used to some extent to regulate surface hardness, which is also affected by entrapped gases and voids, forming porosity. In the region adjacent the core, the molten metal solidifies quickly, as the core acts to remove heat from the casting thereby speeding the cooling in this region. It is difficult to provide feed metal to this portion of the casting due to flow restrictions created by the cores, so that as solidification continues, there is no mechanism to feed this portion of the casting to totally account for shrinkage due to solidification. The result is that the solidified metal at the surface of the part has high strength and quality due to rapid cooling. However, a few thousands of an inch (mils) away from core, 0.030-0.060 inches, there are voids and porosity due to the solidification shrinkage. After the removal of the core from the casting, the aperture is exposed. If the aperture requires machining, a sufficient amount of sound metal must be left to prevent exposure of the voids and porosity. Machining too far into the skin of the piston can expose porosities, which are undesirable in this bearing surface and can eventually lead to premature bearing failure.
What is needed is a method of producing the castings for pistons cheaply while allowing placement of the wrist pin aperture to be varied, so that the same piston castings may be used in compressors with a different strokes. Additionally, the casting should be solidified so that the walls of the piston are fed with molten metal and are not the last portion of the piston to freeze, thereby minimizing casting defects due to void formation and porosity in the piston wall which could be exposed during the machining of the wrist pin bore.